Security experts fear that al Qaeda or a U.S. born "lone wolf" may try to strike back at the U.S. for the death of Osama bin Laden, and in the hours following the announcement authorities at all levels of government were on alert for attacks from bin Laden's loyal and fanatical followers.

"Though bin Laden is dead, al Qaeda is not," said CIA Director Leon Panetta. "The terrorists almost certainly will attempt to avenge him, and we must—and will—remain vigilant and resolute. But we have struck a heavy blow against the enemy."

Sen. Joe Lieberman, I-Conn., chairman of the Senate's Homeland Security Committee, feared the attack could come from a source other than al Qaeda.

"My own great concern in the days ahead is that a so-called 'lone wolf,' a single individual who has been radicalized, will now mobilize himself or herself to take action here at home against the American people," said Lieberman, I-Conn.

Sen. Susan Collins of Maine, the top Republican on the panel, had similar fears.

"One of the concerns that I most have is that a homegrown terrorist will choose this moment to strike in an attempt to retaliate for Osama bin Laden's death," Collins said.

Collins also said she was pleased that the Department of Homeland Security and FBI had released a bulletin Sunday warning police across the U.S. of a possible increase in attacks.

The advisory said that bin Laden's death "could result in retaliatory attacks in the Homeland and against U.S. and Western interests overseas."

"Although soft targets will -- as they have in the past -- remain attractive to homegrown extremist, official targets such as government installations, military personnel and facilities and senior government officials may gain greater prominence," the advisory said.

Former chief of staff to President George W. Bush, Andrew Card, told ABC News that reprisal attacks could already be planned.

"We have to be particularly vigilant about danger that is still out there. This act could cause people to do things people may have planned to do if this day came," Card said.

The Department of Homeland Security said the agency is on full alert.

"Our security posture, which always includes a number of measures both seen and unseen, will continue to protect the American people from an evolving threat picture both in the next days and beyond," said one Department of Homeland Security official.

New York, Los Angeles Police on Alert

Minutes after news of bin Laden's death becoming public, New York City officials placed police on high alert.

"While there is no information indicating a specific threat to New York City, members of the service are reminded to remain alert in the aftermath of the announcement that Osama Bin Laden has been killed," New York Police Commissioner Raymond W. Kelly told the 35,000 officers under his command.

Police in New York and Los Angeles said they would step up patrols near mosques and are warning of lone-wolf style attacks, by single individuals spurred to launch an attack.

In Philadelphia, Police Commissioner Charles Ramsey instructed officers to pay special attention to all mosques and synagogues with hourly checks.

Washington, D.C. police are increasing security along the Metro, the city's subway system, at hotels and at federal buildings.

The State Department told embassy officials and U.S. citizens around the world to be vigilant, particularly in those countries with a large U.S. military presence.

"We have to be on our toes and I have no doubt in my mind the intelligence community and law enforcement community and the FBI are working very hard to ensure this doesn't trigger attacks on Americans or American interests," Card said.

As officials take measures to secure American's safety, experts are warning citizens to be aware of possible security breaches.

Al Qaeda cells have used American passenger jets, including on 9/11 as weapons, and terrorists have attacked hotels, most famously in Mumbai, India, leading some experts to warn travelers to be particularly vigilant.

Travel analyst Joe Brancatelli cautioned travelers to be wary of airlines and hotels and warned of heighten security at airports.